1. Field of the Invention
This invention relates to an optical fiber drawing apparatus, a sealing mechanism for the same, and a method for drawing an optical fiber, and more particularly, to an optical fiber drawing apparatus, a sealing mechanism for the same, and a method for drawing an optical fiber which has simple structure and can maintain the sealing property in a furnace tube even if variation in the outer diameter of the preform is large.
2. Description of the Related Art
FIG. 1 is a schematic illustration showing a conventional optical fiber drawing apparatus (drawing furnace).
An optical fiber drawing furnace 20 mainly comprises a furnace tube 21 to which an optical fiber drawing-preform 1 is inserted, a heater 22 for heating the inserted drawing-preform 1, an upper seal ring 24 and a lower seal ring 25 which prevent inert gas 23 in the furnace tube 21 from leaking to outside.
An optical fiber 2 is obtained by inserting the drawing-preform 1 into the furnace tube 21 made of carbon of the optical fiber drawing furnace 20, heating with the heater 22 to melt the drawing-preform 1, and drawing the melted drawing-preform 1. The drawing-preform 1 is gradually inserted to the optical fiber drawing furnace 20 at a predetermined speed in according with an optical fiber drawing speed and the diameter of the drawing-preform 1, and the inert gas 23 such as helium, argon is flown into the furnace tube 21.
Furnace temperature of the optical fiber furnace 20 reaches high temperature of about 2200° C. Therefore, a mechanism for keeping pressure inside the furnace tube 21 constant and sealing the furnace tube 21 is important to prevent the deterioration or combustion of the furnace tube 21, to stabilize the flow of the inert gas 23 flown into the furnace tube 21, to keep the strength of the drawn optical fiber 2, and to control the fluctuation of the outer diameter of the optical fiber 2.
As the sealing mechanism, the upper seal ring 24 is provided at an upper side of the optical fiber drawing furnace 20. The upper seal ring 24 has a ring shape, and comprises a center hole corresponding to the outer diameter of the drawing-preform 1. The upper seal ring 24 is made of a material such as carbon, carbon felt, and ceramics, in consideration of the heat resistance and the prevention of damage to the drawing perform 1.
As for the conventional upper seal ring 24, a diameter of the center hole (hereafter, referred as “inner diameter”) is fixed in accordance with the outer diameter of the drawing-preform 1. To conduct the drawing by using the sealing mechanism of such shape, it is necessary to reduce the difference (variation) in the outer diameter in a longitudinal direction of the drawing-preform 1 as much as possible. Since the drawing-preform 1 is gradually inserted into the optical fiber drawing furnace 20, the center hole should have a diameter greater than the outer diameter (at a maximum part) of the drawing-preform 1 in the upper seal ring 24. Therefore, a clearance is always generated between the drawing-preform 1 and the upper seal ring 24. In such as case, when an outer diameter changing (increasing or decreasing) part of the drawing-preform 1 passes through the furnace tube 21, amount of the gas discharged from the furnace tube 21 through the clearance will be increased, and change of the pressure in the furnace tube 21 will become greater.
When using the carbon felt composed of carbon fibers for the upper seal ring 24 and the lower seal ring 25 to obtain a high sealing property, the inner diameter of the upper seal ring 24 or the lower seal ring 25 may be increased or reduced to some extent. However, when the fluctuation of the outer diameter of the drawing-preform is greater than the increase or decrease of the inner diameter of the ring, the inner diameter of the ring is extended at the maximum part of the outer diameter of the drawing-preform 1. Since there is also a limit in shrinkage of the ring thereafter, the sealing property will be lost. Further, from the viewpoint of dusting and consumption, there are a lot of disadvantages in the use of the carbon felt.
In the structure where the inert gas 23 is flown into the furnace tube 23 in an upper direction so that the inert gas 23 is discharged to the outside from the furnace tube 21 through the clearance between the drawing-preform 1 and the upper seal ring 24, when the fluctuation of the outer diameter of the drawing-preform 1 in the longitudinal direction is large in the seal ring with the fixed inner diameter, the amount of gas emission varies and the pressure in the furnace tube 21 cannot be kept constant.
Due to the deterioration of the sealing property of the furnace tube 21, the gas pressure in the furnace tube 21 is varied and a flow rate of the gas becomes unstable. As a result, the fluctuation in the outer diameter of the optical fiber is increased, the strength of the optical fiber is decreased, the optical characteristics of the optical fiber are deteriorated, and the production yield is deteriorated. In addition, if an internal pressure of the furnace tube 21 is further decreased, external air of the optical fiber drawing furnace 20 is drawn into the furnace tube 21, so that the drawing work itself will be impossible, and deterioration of the furnace tube 21 will be rapidly accelerated.
To solve such a problem, JP-A-2004-161545 proposes a method for drawing an optical fiber comprising a step of sealing a drawing furnace by blowing the gas while changing an inner diameter of an upper seal ring in accordance with change in the outer diameter of the optical fiber preform which is inserted into the drawing furnace, in order to keep a clearance between the optical fiber preform and the upper seal ring constant even if the outer diameter of the optical fiber perform is changed.
However, according to the conventional method for drawing an optical fiber disclosed by JP-A-2004-161545, the outer diameter in the longitudinal direction of the drawing-preform is previously measured and the inner diameter of the seal ring is varied based on the measured data, to keep the clearance between the drawing-preform and the inner diameter of the seal ring constant. Therefore, complex systems such as outer diameter measuring apparatus for measuring the outer diameter of the preform, driving apparatus for changing the inner diameter of the seal ring, and control mechanism therefor are required, and the problems of installation space and cost due to the enlargement of size of the drawing apparatus may be occurred.